RadioBanter - View Single Post

April 30th 06, 04:00 PM posted to rec.radio.amateur.antenna

wrote:
Tom Donaly wrote:

Consider your normal mobile antenna with a large loading coil. Now,
in your mind's eye, replace the coil with a cylinder. Now, compute the
cutoff frequency for that cylinder for either a TE or TM mode and see
how close you can get to 3.75 Mhz. Of course, if your waves are slow
enough, you should be able to cram something in there, but you have to
show experimentally both that you can do it, and how you can do it.
Tom Donaly, KA6RUH

TE, TM, or TEM a small loading coil cannot behave very much like a
transmission line.

If you read the Corum paper carefully, you see he clearly states it is
an approximation or solution for a coil under the very special
condition of being self-resonant.

He is working on Tesla coils, not loading coils. He is working with
coils that have essentially no termination and that actuallly behave
like a series of L networks with high reactance shunt C and high
reactance series L.

Worse yet, we have one person who is trying to use a large diameter
helice with wide turn spacing that is large enough to support TEM waves
as a comparison to a tiny fraction of a wavelength dieameter and length
inductor that has relatively close turn spacing and tight coupling from
turn to adjacent turn.

Unless we do something to cause the radial electric field to be very
intense and support significant displacement currents, all the standing
waves in the world external to the won't make a coil behave like a
linear conductor.

Notice also how Cecil misquotes to make a point. The Vf I measured on
80 meters for a large bug-catcher style coil was actually .5 compared
to spatial length, not 1.0

On the other hand Cecil has measured virtually nothing, Yuri has
measured nothing, and Harrison probably hasn't even owned a bug catcher
coil being a technician class license holder.

It's easy to dismiss measurements when you have not done a thing on
your own except talk.

73 Tom

Everyone wants to use the most unreliable instrument they possess, their
brain, to measure natural phenomena. The Corum boys wrote an
entertaining paper that makes use of what is evidently an old
technique for explaining helical behavior in the microwave range
to make a point about self-resonant coils being superior to
coil-capacitor combinations for producing long sparks in
Tesla coils. Taken literally, the paper misses the magic
ingredient of validation by experimentation. There are speculative
papers all over the web that do the same thing, but without
experimentation and measurement their most useful purpose is
to provide ideas for people who are willing to do the grunt
work of designing and setting up experiments which show
how good the mental approximations are, their limitations,
and their usefulness. I don't think the Corum paper is
worth much at 3.75 Mhz. Maybe some of the fellows on this
newsgroup can design and carry out an experiment that shows
me wrong. I'd be delighted to be able to use a loading coil
as a low-frequency waveguide. In the meantime, I don't see
any use in Cecil quoting from Corum's paper as if it were
received wisdom while its conclusions are no more proven
than the ideas Cecil is trying to prove.
73,
Tom Donaly, KA6RUH